The invention concerns a method of thermal treatment of fine grain materials.
In chemical and metallurgical process technology there is, from time to time, the need for subjecting fine-grained solid substances to thermal treatment. This may lead to a softening of the substance and/or of its individual phases. The formation of agglomerates and settlements on the walls of the aggregate used for the thermal treatment is undesirable and may negatively impact the product quality and/or impair the course of the process. The agglomerates must subsequently be crushed at great technological expenditures, and the product needs to be reclassified.
An obvious solution for the treatment of fine-grained solid matter is a suspension process in which the fine-grained solid matter is suspended in a gaseous medium. Such gas/solid matter systems are known, for example, as stationary and as circulation fluidized beds. (Fluidized-bed processes for the chemical and metallurgical industries, energy conversion and environmental protection, Chem. -Ing. -Technik 55, 1983, No. 2, pages 87-93). The former work at flow rates below the pneumatic discharge speed to be allocated to the solid matter, the latter, above it; high density of the solid matter is characteristic for both systems, in contrast with the flow reactor which works with low density of the solid matter. If the work is conducted at temperatures within or above the softening range of the solid-matter particles or of individual phases thereof, they will agglomerate even in the fluidized bed and will cake onto the reactor walls.
In DE-OS 3 540 206, a high-temperature suspension process is described for the calcination and for the burning of aluminum hydroxide at 1400 xc2x0 C. which uses a circulation fluidized bed to heat and calcinate the solid matter, and a current reactor to bum the Al2O3 at 1400xc2x0 C. An agglomeration of the solid-matter particles is avoided by feeding the preheated aluminum hydroxide directly into the burner flame, thereby heating it to 1400xc2x0 C. Complete calcination occurs during transport through the current reactor. In addition, in order to prevent agglomerations, the process proceeds at a low density of the solid matter in the reaction zone of the current reactor, and cooling occurs in the upper part of the current reactor by air. Sintering to the oven wall is supposed to be avoided through the high speed of the solid-matter particles relative to the oven wall.
The disadvantage of this method lies in the involved technical apparatus for the heating and the calcination of the solid matter in a circulation fluidized bed and in executing the high-temperature reaction in the downstream current reactor. In spite of low solid-matter density, deposits on the reactor walls due to whirling and reflux cannot be avoided.
Starting from the deficiencies of known solutions, the invention at hand is based on the objective of providing a method and an aggregate for the thermal treatment of fine-grained substances with a grain size of preferably 0 to 1000 xcexcm with a tendency to agglomerate and/or to cake onto the reactor walls at higher temperatures that allows a thermal treatment within the temperature range of their softening and sintering while at the same time precluding the agglomeration of the particles to a large extent and preventing the particles from sintering on the walls of the aggregate.
According to the invention, this objective is attained by a process for thermally treating fine grain materials having an average grain size of approximately 0 to 1000 xcexcm and a tendency towards agglomeration at temperature ranges near and above the agglomeration temperature in a coarse grain extraneous fluidized bed, wherein the temperature range in which the coarse grain shows a tendency towards agglomeration and sintering on the reactor walls due to the particle size and the composition of the material lies above that of the fine grain. The fine grain material may be composed of an abrasive material of corundum or mixtures of corundum and titanium oxide, zirconium oxide, chromium oxide or carbon, while the coarse grain material may be a material similar in nature, or related and/or unrelated and has an average grain size of 1 to 10 mm. The process is executed in an aggregate in the form of a two stage apparatus whose bottom stage is a fluidized bed segment and whose upper stage is a double-walled cooler (upper-oven segment), wherein the interior wall temperatures of the fludized bed segment (20) and the upper oven segment (30) lies at least 10 to 200xc2x0 C., preferably 100xc2x0 C., below the softening and/or agglomeration temperature of the fine grain. The fluidized bed is formed in the fluidized bed segment, an oven space that is closed in itself when completely filled. The fine grain is carried in immediately above the bottom of the fluidized bed, executing a circulating motion and being subjected to a chemical and physical reaction during the thermal treatment, and is carried pneumatically from the fluidized bed into the upper oven segment where it is cooled down to below its softening or agglomeration temperature.